Various types stereo microphones are known such as an MS stereo microphone adding and subtracting side (S) signals to and from mid (M) signals to acquire left (L) and right (R) signals, respectively (see Japanese Unexamined Patent Application Publication No. 2006-174136).
In other words, the L signal corresponds to the sum of the S signal and the M signal, while the R signal corresponds to the subtraction of S signal from the M signal. A microphone unit detecting M signals is composed of a unidirective unit. A microphone unit detecting S signals is composed of a bidirective unit.
The unidirective and bidirective units each having a predetermined size have respective diaphragms and thus different directive axes. The unidirective and bidirective units are disposed vertically or horizontally (front-back direction) to constitute the MS stereo microphone. The respective directive axes of the unidirective and bidirective units are therefore disposed at different vertical or horizontal positions depending on the placement of the units. In such a conventional stereo microphone, the mid unit detecting M signals and the side unit detecting S signals have different directive axes. In other words, the conventional MS stereo microphone includes the mid unit having a different effective acoustic center from that of the side unit.
An increase in such a difference in the positions of the directive axes or in the effective acoustic centers causes misalignment from the original direction of sound pickup axis for detecting sound waves. In particular, short wavelength, i.e., high frequency signals involve this phenomenon remarkably.
Additionally, in the conventional MS stereo microphone, the right and left directive angles is not variable separately.